Apparatus of plural charged-particle beams

ABSTRACT

A multi-beam apparatus for observing a sample with high resolution and high throughput is proposed. In the apparatus, a source-conversion unit forms plural and parallel images of one single electron source by deflecting plural beamlets of a parallel primary-electron beam therefrom, and one objective lens focuses the plural deflected beamlets onto a sample surface and forms plural probe spots thereon. A movable condenser lens is used to collimate the primary-electron beam and vary the currents of the plural probe spots, a pre-beamlet-forming means weakens the Coulomb effect of the primary-electron beam, and the source-conversion unit minimizes the sizes of the plural probe spots by minimizing and compensating the off-axis aberrations of the objective lens and condenser lens.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.16/551,655, filed Aug. 26, 2019, which is a continuation of U.S.application Ser. No. 15/216,258, filed Jul. 21, 2016, now issued as U.S.Pat. No. 10,395,886, which claims the benefit of priority of U.S.provisional application No. 62/195,353 entitled to Ren et al. filed Jul.22, 2015 and entitled “Apparatus of Plural Charged-Particle Beams”. Thedisclosures of the above-referenced applications are incorporated hereinby reference in their entireties.

“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 15/065,342entitled to Weiming Ren et al. filed on Mar. 9, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

This application is related to U.S. application Ser. No. 15/078,369entitled to Weiming Ren et al. filed on Mar. 23, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

This application is related to U.S. application Ser. No. 15/150,858entitled to Xuedong Liu et al. filed on May 10, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

This application is related to U.S. application Ser. No. 15/213,781entitled to Shuai Li et al. filed on Jul. 19, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a charged-particle apparatus with aplurality of charged-particle beams. More particularly, it relates to anapparatus which employs plural charged-particle beams to simultaneouslyacquire images of plural scanned regions of an observed area on a samplesurface. Hence, the apparatus can be used to inspect and/or reviewdefects on wafers/masks with high resolution and high throughput insemiconductor manufacturing industry.

2. Description of the Prior Art

For manufacturing semiconductor IC chips, pattern defects and/oruninvited particles (residuals) inevitably appear on a wafer and/or amask during fabrication processes, which reduce the yield to a greatdegree. To meet the more and more advanced requirements on performanceof IC chips, the patterns with smaller and smaller critical featuredimensions have been adopted. Accordingly, the conventional yieldmanagement tools with optical beam gradually become incompetent due todiffraction effect, and yield management tools with electron beam aremore and more employed. Compared to a photon beam, an electron beam hasa shorter wavelength and thereby possibly offering superior spatialresolution. Currently, the yield management tools with electron beamemploy the principle of scanning electron microscope (SEM) with a singleelectron beam, which therefore can provide higher resolution but can notprovide throughputs competent for mass production. Although a higher andhigher current of the single electron beam can be used to increase thethroughputs, the superior spatial resolutions will be fundamentallydeteriorated by the Coulomb Effect which increases with the beamcurrent.

For mitigating the limitation on throughput, instead of using a singleelectron beam with a large current, a promising solution is to use aplurality of electron beams each with a small current. The plurality ofelectron beams forms a plurality of probe spots on one being-inspectedor observed surface of a sample. The plurality of probe spots canrespectively and simultaneously scan a plurality of small scannedregions within a large observed area on the sample surface. Theelectrons of each probe spot generate secondary electrons from thesample surface where they land on. The secondary electrons comprise slowsecondary electrons (energies ≤50 eV) and backscattered electrons(energies close to landing energies of the electrons). The secondaryelectrons from the plurality of small scanned regions can berespectively and simultaneously collected by a plurality of electrondetectors. Consequently, the image of the large observed area includingall of the small scanned regions can be obtained much faster than thatscanned with a single beam.

The plurality of electron beams can be either from a plurality ofelectron sources respectively, or from a single electron source. For theformer, the plurality of electron beams is usually focused onto andscans the plurality of small scanned regions within a plurality ofcolumns respectively, and the secondary electrons from each scannedregion are detected by one electron detector inside the correspondingcolumn. The apparatus therefore is generally called as a multi-columnapparatus. The plural columns can be either independent or share amulti-axis magnetic or electromagnetic-compound objective lens (such asU.S. Pat. No. 8,294,095). On the sample surface, the beam intervalbetween two adjacent beams is usually as large as 30˜50 mm.

For the latter, a source-conversion unit virtually changes the singleelectron source into a plurality of sub-sources. The source-conversionunit comprises one beamlet-forming (or beamlet-limit) means with aplurality of beam-limit openings and one image-forming means with aplurality of electron optics elements. The plurality of beam-limitopenings divides the primary-electron beam generated by the singleelectron source into a plurality of sub-beams or beamlets respectively,and the plurality of electron optics elements influence the plurality ofbeamlets to form a plurality of parallel (virtual or real) images of thesingle electron source. Each image can be taken as one sub-source whichemits one corresponding beamlet. To make more beamlets available, thebeamlet intervals are at micro meter level. Naturally, one primaryprojection imaging system and one deflection scanning unit within onesingle column are used to project the plurality of parallel images ontoand scan the plurality of small scanned regions respectively. Theplurality of secondary electron beams therefrom is directed by one beamseparator into one secondary projection imaging system, and then focusedby the secondary projection imaging system to be respectively detectedby a plurality of detection elements of one electron detection deviceinside the single column. The plurality of detection elements can be aplurality of electron detectors placed side by side or a plurality ofpixels of one electron detector. The apparatus therefore is generallycalled as a multi-beam apparatus.

The beamlet-forming (or beamlet-limit) means is usually anelectric-conduction plate with through-holes, and a plurality ofthrough-holes therein function the plurality of beam-limit openingsrespectively. Two methods have been used to form the plurality ofparallel images by the image-forming means. For the first one, eachelectron optics element has an electrostatic micro-lens which focusesone beamlet and therefore forms one real image, such as U.S. Pat. No.7,244,949. For the second one, each electron optics element has aelectrostatic micro-deflector which deflects one beamlet and thereforeforms one virtual image, such as U.S. Pat. No. 6,943,349 and first crossreference. The concept of using an electrostatic deflector to form avirtual image of an electron source was used in the famous two-slitelectron interference experiments as early as in 1950s (FIG. 1 of thepaper “The Merli-Missiroli-Pozzi Two-Slit Electron-InterferenceExperiment” published in Physics in Perspective, 14 (2012) 178-195 byRodolfo Rosa). Coulomb Effect in the second method is weaker than in thefirst method due to one real image has a higher current density, andhence the second method is more advantageous for achieving both highthroughput and high resolution.

To reduce aberrations of the plurality of probe spots, the primaryprojection imaging system basically comprises one transfer lens and oneobjective lens, and the transfer lens bends the plurality of beamlets topass through the objective lens as close to the optical axis thereof aspossible. For a source-conversion unit with the second method, thebending function of the transfer lens can be done by the plurality ofmicro-deflectors, and therefore the transfer lens can be removed, asproposed in the first cross reference and shown in FIG. 1 . Without thetransfer lens, the projection imaging system will be simplified and easyin manufacturing and operation.

In FIG. 1 , the electron source 101 on the primary optical axis 100_1generates the primary electron beam 102 with a source crossover (virtualor real) 101 s. The condenser lens 110 focuses the primary-electron beam102 incident onto the source-conversion unit 120 with a desired currentdensity. The peripheral electrons of the primary electron beam 102 arecut off by the main opening of the main aperture plate 171, which canalso be placed above the condenser lens 110. Three beamlets (102_1,102_2 and 102_3) of the primary-electron beam 102 are respectivelydeflected towards the primary optical axis 100_1 by the threemicro-deflectors (122_1, 122_2 and 122_3) of the image-forming mean 122,and pass through three beam-limit openings (121_1, 121_2 and 121_3) ofthe beamlet-limit means 121. Then, the three virtual images (102_1 v,102_2 v and 102_3 v) formed by the deflected three beamlets areprojected by the objective lens 131 onto the surface 7 of the sample 8and three probe spots (102_1 s, 102_2 s and 102_3 s) therefore areformed thereon.

If the three beamlets are deflected close to or passing through thefront focal point of the objective lens 131, they will perpendicularlyland on the sample surface 7 and aberrations of the off-axis probe spots(such as 102_2 s) due to the objective lens 131 will decrease to a greatdegree. However, in this case, the deflection angles of the threebeamlets become larger, which not only require stronger deflectionpowers of the three micro-deflectors but also generate larger deflectionaberrations. The first issue may incur electric breakdown of the threemicro-deflectors, and the second issue may enlarge the sizes of theoff-axis probe spots to an unacceptable level.

The beam-limit openings limit the currents of the three probe spots, andthe currents are changed by tuning the focusing power of the condenserlens 110 to vary the current density of the primary electron beam 102.For the three micro-deflectors, the incident angles of the threebeamlets change with the focusing power, and the deflection powersthereof need adjusting accordingly. The time and the effort for changingobserving conditions are the less the better.

Accordingly, it is necessary to provide a multi-beam apparatus which hasno or fewer foregoing issues, and therefore can provide high imageresolution and high throughput. Especially, a multi-beam apparatus whichcan inspect and/or review defects on wafers/masks with high resolutionand high throughput is needed to match the roadmap of the semiconductormanufacturing industry.

SUMMARY OF THE INVENTION

The object of this invention is to provide a new multi-beam apparatuswhich provide both high resolution and high throughput for observing asample in flexibly varying observing conditions (such as currents andlanding energies of the probe spots, electrostatic field on the samplesurface). The apparatus can function as a yield management tool toinspect and/or review defects on wafers/masks in semiconductormanufacturing industry.

In the apparatus, one condenser collimates or substantially collimatesthe primary-electron beam into one source-conversion unit, thesource-conversion unit deflects a plurality of beamlets of theprimary-electron beam towards the optical axis of one objective lens,and the objective lens focuses the plurality of deflected beamlets ontoone being-observed surface of one sample and therefore a plurality ofprobe spots is formed thereon, wherein the deflection angles of theplurality of deflected beamlets are adjusted to reduce the sizes of theplurality of probe spots. The currents of the plurality of probe spotscan be varied by changing either or both of the focusing power and theposition of the first principal plane of the condenser lens. Thesource-conversion unit can further reduce the sizes and the sizedifferences of the plurality of probe spots by compensating off-axisaberrations thereof. Furthermore, to weaken the Coulomb effect due tothe primary-electron beam as much as possible, the beamlet-forming meansof the source conversion unit can be placed close to the single electronsource, a pre-beamlet-forming means can be employed close to the singleelectron source.

Accordingly, the invention therefore provides a multi-beam apparatus,which comprises an electron source, a condenser lens below the electronsource, a source-conversion unit below the condenser lens, an objectivelens below the source-conversion unit, a deflection scanning unit belowthe source-conversion unit, a sample stage below the objective lens, abeam separator below the source-conversion unit, a secondary projectionimaging system, and an electron detection device with a plurality ofdetection elements. The electron source, the condenser lens and theobjective lens are aligned with a primary optical axis of the apparatus,and the sample stage sustains the sample so that the surface faces tothe objective lens. The source-conversion unit comprises abeamlet-forming means with a plurality of beam-limit openings and animage-forming means with a plurality of electron optics elements. Theelectron source generates a primary-electron beam along the primaryoptical axis, and the primary-electron beam is focused by the condenserlens to become a substantially parallel beam and then incident into thesource-conversion unit. A plurality of beamlets of the primary-electronbeam exits from the source-conversion unit, which respectively passesthrough the plurality of beam-limit openings and is deflected by theplurality of electron optics elements towards the primary optical axis,and deflection angles of the plurality of beamlets are different. Theplurality of beamlets, focused by the objective lens onto the surfaceand forms a plurality of probe spots thereon, is deflected by thedeflection scanning unit to scan the plurality of probe spotsrespectively over a plurality of scanned regions within an observed areaon the surface, and currents of the plurality of probe spots are limitedby the plurality of beam-limit openings. A plurality of secondaryelectron beams, generated by the plurality of probe spots respectivelyfrom the plurality of scanned regions and directed into the secondaryprojection imaging system by the beam separator, is focused by thesecondary projection imaging system to keep the plurality of secondaryelectron beams to be detected by the plurality of detection elementsrespectively, and each detection element therefore provides an imagesignal of one corresponding scanned region.

In one embodiment, the deflection angles can be individually set toreduce aberrations of the plurality of probe spots respectively. Theplurality of electron optics elements is below and aligned with theplurality of beam-limit openings respectively. Each of the plurality ofelectron optics elements can be a 4-pole lens. Currents of the pluralityof probe spots are varied by using the condenser lens to change acurrent density of the primary-electron beam incident into thesource-conversion unit. The apparatus may further comprise apre-beamlet-forming means with a plurality of beamlet-forming aperturesabove the source-conversion unit, wherein the plurality of beamletspasses through the plurality of beamlet-forming apertures respectivelyand most of electrons outside the plurality of beamlets are cut off.

The plurality of electron optics elements, in this embodiment, cancompensate one or up to all of field curvature, astigmatism anddistortion aberrations of the plurality of probe spots to further reducesizes and distortions thereof. Each of the plurality of electron opticselements can be an 8-pole lens. Each of the plurality of electron opticselements may comprise one micro-lens and two 4-pole lenses which arealigned with and placed along an optical axis of the each element, andthe two 4-pole lenses have a 45° difference in azimuth. For that each ofthe plurality of electron optics elements, one of the two 4-pole lensesis on a beamlet exit side and one corresponding beamlet is deflected bythe one 4-pole lens.

The condenser lens, in one embodiment, may comprise multiple annularelectrodes which are placed at different axial positions along andaligned with the primary optical axis, and voltages thereof can beadjusted to change the current density. The condenser lens, in anotherembodiment, may comprise at least two single magnetic lenses which areplaced at different axial positions along and aligned with the primaryoptical axis, and excitations thereof can be adjusted to change thecurrent density of the primary-electron beam incident into thesource-conversion unit. The condenser lens, in still another embodiment,may comprise multiple annular electrodes and at least one singlemagnetic lens which are placed at different axial positions along andaligned with the primary optical axis, and voltages of the electrodesand excitations of the at least one single magnetic lens can be adjustedto change the current density.

Landing energies of the plurality of beamlets on the surface are variedby changing a potential thereof.

The present invention also provides a multi-beam apparatus, whichcomprises an electron source, a condenser lens below the electronsource, a source-conversion unit below the condenser lens, an objectivelens below the source-conversion unit, a deflection scanning unit belowthe source-conversion unit, a sample stage below the objective lens, abeam separator below the source-conversion unit, a secondary projectionimaging system, and an electron detection device with a plurality ofdetection elements. The electron source, the condenser lens and theobjective lens are aligned with a primary optical axis of the apparatus,and the sample stage sustains the sample so that the surface faces tothe objective lens. The source-conversion unit comprises abeamlet-forming means with a plurality of beam-limit openings and animage-forming means with a plurality of electron optics elements. Theelectron source generates a primary-electron beam along the primaryoptical axis, which is focused by the condenser lens, and then isincident into the source-conversion unit with a convergent or divergentangle. A plurality of beamlets of the primary-electron beam exits fromthe source-conversion unit, respectively passes through the plurality ofbeam-limit openings, and is deflected by the plurality of electronoptics elements towards the primary optical axis. The plurality ofbeamlets is focused by the objective lens onto the surface and forms aplurality of probe spots thereon. Deflection angles of the plurality ofbeamlets are individually set to reduce aberrations of the plurality ofprobe spots respectively, and the deflection scanning unit deflects theplurality of beamlets to scan the plurality of probe spots respectivelyover a plurality of scanned regions within an observed area on thesurface. Currents of the plurality of probe spots are limited by theplurality of beam-limit openings. A plurality of secondary electronbeams, generated by the plurality of probe spots respectively from theplurality of scanned regions and directed into the secondary projectionimaging system by the beam separator. The secondary projection imagingsystem focuses and keeps the plurality of secondary electron beams to bedetected by the plurality of detection elements respectively, and eachdetection element therefore provides an image signal of onecorresponding scanned region.

The plurality of electron optics elements could compensate one or up toall of field curvature, astigmatism and distortion aberrations of theplurality of probe spots to further reduce sizes and distortionsthereof. Currents of the plurality of probe spots are varied by usingthe condenser lens to adjust a current density of the primary-electronbeam incident into the source-conversion unit. The plurality of electronoptics elements may be below the plurality of beam-limit openings. Theapparatus may further comprise a pre-beamlet-bending means with aplurality of pre-bending micro-deflectors respectively above theplurality of beam-limit openings. The plurality of pre-bendingmicro-deflectors may deflect the plurality of beamlets to beperpendicularly incident into the plurality of beam-limit openings. Theapparatus may further comprise a pre-beamlet-forming means with aplurality of beamlet-forming apertures above the source-conversion unit,wherein the plurality of beamlets passes through the plurality ofbeamlet-forming apertures respectively and most of electrons outside theplurality of beamlets are cut off.

The present invention also provides a multi-beam apparatus, whichcomprises an electron source, a beamlet-forming plate providing aplurality of beam-limit openings below the electron source, a condenserlens below the beamlet-forming plate, a plurality of electron opticselements below the condenser lens, an objective lens below the pluralityof electron optics elements, a deflection scanning unit below theplurality of electron optics elements, a sample stage below theobjective lens, a beam separator below the plurality of electron opticselements, a secondary projection imaging system, and an electrondetection device with a plurality of detection elements. The electronsource, the condenser lens and the objective lens are aligned with aprimary optical axis of the apparatus, and the sample stage sustains thesample so that the surface faces to the objective lens. The electronsource generates a primary-electron beam along the primary optical axis,which is trimmed by the beamlet-forming plate into a plurality ofbeamlets respectively passing through a plurality of through-holes of afirst group therein, and the plurality of through-holes functions as aplurality of beam-limit openings of the apparatus. The condenser lensfocuses the plurality of beamlets to be deflected by the plurality ofelectron optics elements respectively towards the primary optical axis.The plurality of beamlets is focused by the objective lens onto thesurface and forms a plurality of probe spots thereon, and deflectionangles of the plurality of beamlets are individually set to reduceaberrations of the plurality of probe spots respectively. The deflectionscanning unit deflects the plurality of beamlets to scan the pluralityof probe spots respectively over a plurality of scanned regions withinan observed area on the surface, and currents of the plurality of probespots are limited by the plurality of beam-limit openings. A pluralityof secondary electron beams is generated by the plurality of probe spotsrespectively from the plurality of scanned regions and directed into thesecondary projection imaging system by the beam separator. The secondaryprojection imaging system focuses and keeps the plurality of secondaryelectron beams to be detected by the plurality of detection elementsrespectively, and each detection element therefore provides an imagesignal of one corresponding scanned region.

The plurality of beamlets is perpendicularly incident into the pluralityof electron optics elements. The plurality of electron optics elementscompensates one or up to all of field curvature, astigmatism anddistortion aberrations of the plurality of probe spots to further reducesizes and distortions thereof. Currents of the plurality of beamlets, inone embodiment, can be varied by adjusting an angular intensity of theelectron source. Currents of the plurality of beamlets, in anotherembodiment, can be varied by changing radial sizes of the plurality ofbeam-limit openings. The radial sizes are changed by moving thebeamlet-forming plate to locate a plurality of through-holes of a secondgroup therein as the plurality of beam-limit openings.

The present invention also provides a method to form a plurality ofprobe spots in a SEM, which comprises steps of generating aprimary-electron beam by an electron source, collimating orsubstantially collimating the primary-electron beam by a condenser lens,trimming the collimated primary-electron beam into a plurality ofbeamlets by a first plate with first through-holes, deflecting theplurality of beamlets towards an optical axis of an objective lens withdifferent deflection angles by a plurality of electron optics elements,and focusing the plurality of deflected beamlets onto a being-observedsurface of a sample by the objective lens, wherein the plurality ofdeflected and focused beamlets forms the plurality of probe spots.

The method may further comprise a step of individually setting thedeflection angles to reduce aberrations of the plurality of probe spotsrespectively. The method may further comprise a step of compensating oneor up to all of field curvature, astigmatism and distortion aberrationsof the plurality of probe spots by the plurality of electron opticselements. The method may further comprise a step of varying a currentdensity of the collimated primary-electron beam by moving a firstprincipal plane of the condenser lens. The method may further comprise astep of cutting off most of electrons outside the plurality of beamletsby a second plate with second through-holes before the trimming step.

The present invention also provides a method to form a plurality ofprobe spots in a SEM, which comprises generating a primary-electron beamby an electron source, trimming the primary-electron beam into aplurality of beamlets by a plate with a plurality of through-holes,focusing the plurality of beamlets by a condenser lens, deflecting theplurality of beamlets towards an optical axis of an objective lens by aplurality of electron optics elements, focusing the plurality ofdeflected beamlets onto a being-observed surface of a sample by theobjective lens, wherein the plurality of deflected and focused beamletsforms the plurality of probe spots, and setting deflection angles of theplurality of deflected beamlets individually to reduce aberrations ofthe plurality of probe spots respectively.

The method may further comprise a step of compensating one or up to allof field curvature, astigmatism and distortion aberrations of theplurality of probe spots by the plurality of electron optics elements.The method may further comprise a step of varying currents of theplurality of beamlets by changing an angular intensity of the electronsource. The method may further comprise a step of changing currents ofthe plurality of beamlets by using another plurality of through-holes ofthe plate in the trimming step.

The present invention also provides a device for providing multiplesources, which comprises a charged-particle source for providing aprimary beam along an optical axis of the device, means forsubstantially collimating the primary beam, and means for imaging aplurality of virtual images of the charged-particle source with aplurality of beamlets of the collimated primary beam, wherein theplurality of virtual images becomes the multiple sources which emit theplurality of beamlets respectively.

The device may further comprise means for varying currents of theplurality of beamlets. The device may further comprise means forsuppressing Coulomb effect due to the primary beam.

The present invention also provides a multi-beam apparatus, whichcomprises the device for providing the multiple sources, means forprojecting the plurality of virtual images on a sample surface such thata plurality of probe spots is formed thereon, means for scanning theplurality of probe spots on the sample surface, and means for receivinga plurality of signal particle beams generated from the sample surfacedue to plurality of probe spots. The multi-beam apparatus may furthercomprise means for individually deflecting the plurality of beamlets toreduce aberrations of the plurality of probe spots respectively. Themulti-beam apparatus may further comprise means for individuallycompensating the aberrations of the plurality of probe spots. Theprojecting means is a single objective lens.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a schematic illustration of a conventional multi-beamapparatus disclosed in the first cross reference.

FIG. 2 is a schematic illustration of one configuration of a newmulti-beam apparatus in accordance with one embodiment of the presentinvention.

FIG. 3 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 4 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 5 is a schematic illustration of one configuration of themicro-deflector-and-compensator array in FIG. 4 in accordance withanother embodiment of the present invention.

FIG. 6A is a schematic illustration of one configuration of themicro-deflector-and-compensator array in FIG. 4 in accordance withanother embodiment of the present invention.

FIGS. 6B-6D are schematic illustrations of one example of theconfiguration of the micro-deflector-and-compensator array in FIG. 6A inaccordance with another embodiment of the present invention.

FIG. 7A and FIG. 7B are respectively a schematic illustration of oneconfiguration of the micro-deflector-and-compensator array in FIG. 4 inaccordance with another embodiment of the present invention.

FIG. 8 is a schematic illustration of one operation mode of theapparatus in FIG. 3 .

FIG. 9 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 10 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 11A and FIG. 11B are respectively a schematic illustration of oneoperation mode of the apparatus in FIG. 10 .

FIG. 12 , FIG. 13A and FIG. 13B are respectively a schematicillustration of one embodiment of the movable condenser lens in FIG. 10.

FIG. 14 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 15A is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 15B is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIG. 16 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not used to limit thepresent invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of anelectron optics element (such as a round lens or a multipole lens), oran apparatus”, “radial” means “in a direction perpendicular to theoptical axis”, “on-axial” means “on or aligned with the optical axis”,and “off-axis” means “not on or not aligned with the optical axis”.

In this invention, X, Y and Z axe form Cartesian coordinate, the opticalaxis is on the Z-axis and a primary-electron beam travels along theZ-axis.

In this invention, “primary electrons” means “electrons emitted from anelectron source and incident onto a being-observed or inspected surfaceof a sample, and “secondary electrons” means “electrons generated fromthe surface by the “primary electrons”

In this invention, all terms relate to through-holes, openings andorifices mean openings or holes penetrated through one plate.

In the new multi-beam apparatus, the primary-electron beam is focusedparallel or substantially parallel into one source-conversion unit byone condenser. A plurality of beamlets of the primary-electron beam isat first deflected by the source-conversion unit towards the opticalaxis of one objective lens, then focused by the objective lens onto thesample surface, and finally forms a plurality of probe spots thereon.The deflection angles of the plurality of deflected beamlets are set tominimize the off-axis aberrations due to the objective lens. Thecurrents of the plural probe spots can be varied by changing either orboth of the focusing power and the position of the first principal planeof the condenser lens, and the sizes and their size differences of theplural probe spots can be further reduced by compensating the residualoff-axis aberrations by the source-conversion unit. In addition, for theplural probe spots, the blurs due to strong Coulomb effect of theprimary-electron beam can be reduced by placing the beamlet-formingmeans of the source-conversion unit close to the single electron sourceor additionally using one pre-beamlet-forming means above thesource-conversion unit.

Next, some embodiments of the new apparatus will be described. For sakeof clarity, only three beamlets are shown and the number of beamlets canbe anyone. The deflection scanning unit, the beam separator, thesecondary projection imaging system and the electron detection device inprior art can be used here, and for sake of simplification, they are notshown or even not mentioned in the illustrations and the description ofthe embodiments.

One embodiment 200A of the new apparatus is shown in FIG. 2 . In FIG. 2, the election source 101, the main opening of the main aperture plate271, the condenser 210, the source-conversion unit 220 and the objectivelens 131 are aligned with the primary optical axis 200_1 of theapparatus. The electron source 101 generates a primary-electron beam 102along the primary optical axis 200_1 and with a source crossover(virtual or real) 101 s, the condenser lens 210 focuses theprimary-electron beam 102 to become a parallel beam along the primaryoptical axis 200_1 and perpendicularly incident onto thesource-conversion unit 220. In the source-conversion unit 220, threebeamlets 102_1, 102_2 and 102_3 of the primary-electron beam 102 arerespectively deflected by three micro-deflectors 222_1 d, 222_2 d and222_3 d of the image-forming means 222 towards the primary optical axis200_1 and pass through three beam-limit openings 221_1, 221_2 and 221_3of the beamlet-limit means 221. The three beam-limit openings limitcurrents of the three deflected beamlets. The objective lens 131 focusesthe three deflected beamlets onto the surface 7 of the sample 8, andaccordingly generates three images 102_1 s, 102_2 s and 102_3 s of thesource crossover lots thereon. Each image forms one probe spot on thesurface 7, and the three images are also called as three probe spots102_1 s, 102_2 s and 102_3 s. The deflection angles of the threedeflected beamlets are set to minimize the off-axis aberrations of thethree images due to the objective lens 131, wherein the three deflectedbeamlets typically pass through or approach the front focal point of theobjective lens 131. The main aperture plate 271 cuts the peripheralelectrons of the primary-electron beam 102 to reduce the Coulomb Effectthereof as much as possible.

The beamlet-forming means 221 can be an electric-conduction plate withthrough-holes, and three through-holes therein function as the threebeam-limit openings (221_1˜221_3) respectively. In FIG. 2 , the threedeflected beamlets (102_1˜102_3) do not perpendicularly pass through thethree beam-limit openings (221_1˜221_3), and therefore suffers electronscatterings to a certain degree related to the deflection angles. Thescattering electrons in each beamlet will enlarge the probe spot and/orbecome a background noise and therefore deteriorate the image resolutionof the corresponding scanned region. To avoid this issue, the currentsof the three beamlets can be cut when the three beamlets are parallel tothe primary optical axis 200_1. Accordingly, another embodiment 300A ofthe new apparatus is proposed in FIG. 3 . In comparison with theembodiment 200A in FIG. 2 , the beamlet-limit means 221 is placed abovethe image-forming means 222 and renamed as beamlet-forming means 321 inthe source-conversion unit 320 in FIG. 3 . The three beam-limit openings321_1, 321_2 and 321_3 of the beamlet-forming means 321 are respectivelyaligned with the three micro-deflectors 222_1 d, 222_2 d and 222_3 d ofthe image-forming means 222, and the three beamlets 102_1, 102_2 and102_3 are perpendicularly incident into the three beam-limit openingsand the three micro-deflectors in succession.

As well known, the condenser lens 210 and the objective lens 131generate off-axis aberrations (such as field curvature, astigmatism anddistortion) which enlarge the sizes and/or influence the positions ofthe probe spots formed by those off-axis beamlets (not along the primaryoptical axis of the apparatus). As mentioned above, the off-axisaberrations due to the objective lens 131 have been minimized byindividually optimizing the trajectories of the off-axis beamlets (i.e.appropriately setting the deflection angles thereof). To further reducethe sizes and size differences of the probe spots, the off-axisaberrations due to the condenser lens 210 and the residual off-axisaberrations due to the objective lens 131 have to be compensated.Accordingly another embodiment 400A of the new apparatus is proposed inFIG. 4 , wherein the image-forming means 422 has threemicro-deflector-and-compensator elements 422_1 dc, 422_2 dc and 422_3dc. Each micro-deflector-and-compensator element is aligned with one ofthree beam-limit openings 321_1, 321_2 and 321_3 of the beamlet-formingmeans 321, functions as one micro-deflector to deflect one beamlet andone micro-compensator to compensate the field curvature, astigmatism anddistortion of the corresponding probe spot.

Each of three micro-deflectors (222_1 d˜222_3 d) in FIG. 2 and FIG. 3can simply be formed by a dipole lens whose two electrodes is orientedto generate a dipole field in the required deflection direction of thecorresponding beamlet, or a quadrupole or 4-pole lens whose fourelectrodes can generate a dipole field in any desired direction. In thelater case, all the micro-deflectors can be configured to be same instructure and orientation. This is advantageous from the manufacturingpoint of view.

In FIG. 4 , each of three micro-deflector-and-compensator elements(422_1 dc˜422_3 d) can simply be formed by a 4-pole lens whose fourelectrodes can generate a dipole field in any desired direction, around-lens field and a quadrupole field in the required compensationdirection of the corresponding probe spot, or a octupole or 8-pole lenswhose eight electrodes can generate a round-lens field, a dipole fieldand a quadrupole field both in any desired direction. To generate around-lens field, the inner surfaces of the four or the eight electrodesform a circular shape in a radial cross-section, as shown in FIG. 5 . Inthe later case, all the micro-deflector-and-compensator elements can beconfigured to be same in structure and orientation. This is advantageousfrom the manufacturing point of view.

To generate all the foregoing fields, the voltages of the electrodes inone 4-pole lens or 8-pole lens are different and may be high enough toincur electric breakdown. To avoid this issue, eachmicro-deflector-and-compensator element can be formed by two or moremicro-multipole-lenses, or one or more micro-multipole-lenses and one ormore micro-lens. In addition, to reduce aberrations due to eachmicro-deflector-and-compensator element, the corresponding beamlet isbetter passing through the round-lens field and the quadrupole fieldalong the optical axis thereof, i.e. the off-axis aberrationcompensation is better done before the beamlet deflection. Hence thedipole field is better generated by the micro-multipole-lens on thebeamlet exit side of each micro-deflector-and-compensator element.Accordingly, FIG. 6A shows such an embodiment of the image-forming means422 in FIG. 4 , wherein each of three micro-deflector-and-compensatorelements (422_1 dc˜422_3 dc) is formed by one micro-lens in the firstlayer 422-1, one micro-multipole-lens in the second layer 422-2 and onemicro-multipole-lens in the third layer 422-3, and the micro-lens andthe two micro-multipole-lenses are aligned with its optical axis. Forexample, the micro-lens 422-1_2 and two micro-multipole-lenses 422-2_2and 422-3_2 form the micro-deflector-and-compensator element 422_2 dcand are aligned with the optical axis 422_2 dc_1 thereof.

In each micro-deflector-and-compensator element in FIG. 6A, the twomicro-multipole-lenses can respectively be a dipole lens and a 4-polelens, or a dipole lens and an 8-pole lens, or a 4-pole lens, etc. FIGS.6B, 6C and 6D show one example, wherein each micro-lens is formed by oneannular electrode with a round inner surface, each micro-multipole-lensis a 4-pole lens, and each 4-pole lens in the second layer 422-2 and thecorresponding 4-pole lens in the third layer 422-3 have a 45° differencein azimuth or orientation. For each micro-deflector-and-compensatorelement, the micro-lens generates the round-lens field, the two 4-polelenses generate the quadrupole field, and the dipole field is bettergenerated by the 4-pole lens in the third layer 422-3.

To operate one micro-lens-and-compensator element in FIG. 4 , adriving-circuit needs connecting with each electrode thereof. To preventthe driving-circuits from being damaged by the beamlets 102_1˜102_3, theimage-forming means 422 can comprises one electric-conductioncover-plate which has a plurality of through-holes and is placed abovethe electrodes of all the micro-lens-and-compensator elements. Eachthrough-hole is for the corresponding beamlet passing through. Theforegoing fields of each micro-lens-and-compensator element are betterwithin a limited range so as to avoid influencing the adjacent beamlets.Therefore it is better to use two electric-conduction shielding-platesto sandwich the electrodes of all the micro-lens-and-compensatorelements, wherein each shielding-plate has a plurality of through-holesfor the beamlets passing through. FIG. 7A shows one way to implement theforegoing measures in the embodiment in FIG. 6A.

In FIG. 7A, the first-upper and the first-lower electric-conductionplates 422-1-CL1 and 422-1-CL2 are respectively placed above and belowthe micro-lens 422-1_1, 422-1_2 and 422-1_3 in the first layer 422-1.The first-upper and the first-lower insulator plates 422-1-IL1 and422-1-IL2, respectively with three first-upper and first-lower orificesfor beamlets passing through, support the micro-lenses 422-1_1, 422-1_2and 422-1_3 and therefore make the first layer 422-1 more stable inconfiguration. Similarly, in the second layer 422-2, the second-upperand the second-lower electric-conduction plates 422-2-CL1 and 422-2-CL2are respectively placed above and below the micro-multipole-lenses422-2_1, 422-2_2 and 422-2_3. The second-upper and the second-lowerinsulator plates 422-2-IL1 and 422-2-IL2, respectively with threesecond-upper and second-lower orifices for beamlets passing through,support the micro-lenses 422-2_1, 422-2_2 and 422-2_3 and therefore makethe second layer 422-1 more stable in configuration. In the third layer422-3, the third-upper and the third-lower electric-conduction plates422-3-CL1 and 422-3-CL2 and the third-upper and the third-lowerinsulator plates 422-3-IL1 and 422-3-IL2 function the same ways as theircounterparts in the second layer 422-2.

In each layer in FIG. 7A, the radial dimensions of the through-holes arepreferred smaller than the radial dimensions of the orifices so as toavoid charging-up on the inner sidewalls thereof, and smaller than theinner radial dimensions of the electrodes of themicro-lens/micro-multipole-lens so as to more efficiently reduce thefields leaking out. To reduce the possibility of beamlet incurringelectron scattering, each through-hole in the first-upperelectric-conduction plate is preferred in an upside-down funnel shape(i.e. the small end is on the beamlet incident side thereof). Thebeamlet-forming means 321 can be an electric-conduction plate withthrough-holes, and three through-holes therein functions as the threebeam-limit openings (321_1˜321_3) respectively. Therefore thebeamlet-forming means 321 can be combined with the embodiment of theimage-forming means 422 in 7A for simplifications in structure andmanufacturing. In FIG. 7B, the beamlet-forming means 321 and thefirst-upper electric-conduction plate 422-1-CL1 are combined, thefirst-lower electric-conduction plate 422-1-CL2 and the second-upperelectric-conduction plate 422-2-CL1 are combined, and the second-lowerelectric-conduction plate 422-2-CL2 and the third-upperelectric-conduction plate 422-3-CL1 are combined.

For the foregoing embodiments of the new apparatus in FIGS. 2, 3 and 4 ,the currents of the probe spots 102_1 s˜102_3 can be varied within asmall range by adjusting the focusing power of the condenser lens 210 tomake the primary-electron beam 102 slightly divergent or convergent.FIG. 8 shows one divergent mode in the embodiment 300 in FIG. 3 . In onedivergent mode, the current density of the primary-electron beam 102 issmaller than that in the parallel mode in FIG. 3 , and therefore thecurrents of the three beamlets below the beamlet-forming means 321 arereduced. In one divergent/convergent mode of the embodiments in FIG. 3and FIG. 4 , the three beamlets will not perpendicularly pass throughthe three beam-limit openings (321_1˜321_3), and therefore sufferselectron scatterings to a certain degree. To avoid this issue, onepre-beamlet-bending means can be placed above the beamlet-forming means321 of the source-conversion unit 320 in FIG. 3 or 420 in FIG. 4 , whichcomprises three pre-bending micro-deflectors to respectively deflect thethree beamlets perpendicularly passing through the three beam-limitopenings. FIG. 9 shows how to implement this way in the embodiment 300Ain FIG. 3 , and in one divergent mode how to operate three pre-bendingmicro-deflectors 523_1 d, 523_2 d and 523_3 d of the pre-beamlet-bendingmeans 523 of the new source-conversion unit 520 in the correspondingembodiment 500A.

For the foregoing embodiments of the new apparatus in FIGS. 2, 3 and 4 ,the currents of the probe spots 102_1 s˜102_3 can be varied within alarge range by moving the first principal plane of the condenser 210 andaccordingly adjusting the focusing power of the condenser lens 210 tomake the primary-electron beam 102 become a parallel beam, i.e. thefirst principal plane of the condenser lens 210 is movable along theprimary optical axis of the new apparatus. When the first principalplane is closer to the electron source 101, the primary-electron beam102 is focused earlier and with a higher current density, andaccordingly the currents of the three beamlets below the beamlet-formingmeans 321 are increased. The closer to the electron source 101 the firstprincipal plane is, the higher the currents are, and vice versa. Henceas the first principal plane is moved along the primary optical axis,the currents of the three probe spots change accordingly and the threebeamlets keep perpendicularly passing through the three beam-limitopenings.

FIG. 10 shows one embodiment 600A which uses a movable condenser lens610 to replace the condense lens 210 in the embodiment 300A in FIG. 3 ,wherein the first principal plane 610_2 is at position P1 and can bemoved along the primary optical axis 600_1 of the apparatus. In FIG. 11Athe first principal plane 610_2 is moved from the position P1 to theposition P2 further away from the electron source 101, and accordinglythe currents of the beamlets 102_1, 102_2 and 102_3 decrease. In FIG.11B the first principal plane 610_2 is moved from the position P1 to theposition P3 closer to the electron source 101, and accordingly thecurrents of the beamlets 102_1, 102_2 and 102_3 increase. Due toprimary-electron beam 102 is kept as a parallel beam when varying thecurrents of the beamlets, the deflection angles thereof do not need tobe re-tuned. This will avoid the time for adjusting themicro-deflectors.

To extend the current variant range, the primary-electron beam 102 inFIG. 11A can be weakly focused so as to keep a divergence, and theprimary-electron beam 102 in FIG. 11B can be strongly focused to becomea convergent beam. As well known, the size of each probe spot isdetermined by the Gaussian image size of the source crossover lots, thegeometrical aberrations, the diffraction effect and the Coulomb Effect,and the size can be minimized by balancing these blurs. Adjusting theposition of the first principal plane 610_2 of the movable condenserlens 610 will break the balance to a certain degree, and therefore thesize of each probe spot may increase when the current thereof ischanged. When changing the position of the first principal plane 610_2,slightly remaining an appropriate divergence or convergence of theprimary-electron beam 102 can weaken the size increasing of the probespots.

The displacement of the first principal plane 610_2 can be done bymechanically moving the position of the movable condenser lens 610 orelectrically moving the position and/or changing the shape of theround-lens field thereof. The movable condenser lens 610 can beelectrostatic, or magnetic, or electromagnetic compound. FIG. 12 showsone electrostatic embodiment 610 e of the movable condenser lens 610,and shapes of the primary-electron beam 102 when the first principalplane 610 e_2 is at the positions P1, P2 and P3 respectively. Themovable condenser lens 610 e is an electrostatic lens with four annularelectrodes 610 e-e 1, 610 e-e 2, 610 e-e 3 and 610 e-e 4 aligned withits optical axis 610 e_1. The optical axis 610 e_1 is placed coincidentwith the primary optical axis 600_1. The focusing power and the positionof the first principal plane 610 e_2 of the embodiment 610 e vary withthe excitation mode of the annular electrodes 610 e-e 1-610 e-e 4. Whenthe electrodes 610 e-e 1, 610 e-e 2 and 610 e-e 4 are at samepotentials, by appropriately setting the potential of the electrode 610e-e 3, the first principal plane 610 e_2 will be at the position P2close to the electrode 610 e-e 3 and the primary-electron beam 102 canbe collimated over there. When the electrodes 610 e-e 1, 610 e-e 3 and610 e-e 4 are at same potentials, by appropriately setting the potentialof the electrode 610 e-e 2, the first principal plane 610 e_2 will be atthe position P3 close to the electrode 610 e-e 2 and theprimary-electron beam 102 can be collimated over there. When theelectrodes 610 e-e 1 and 610 e-e 4 are at same potentials, byappropriately setting the potentials of the electrodes 610 e-e 2 and610-e 3, the first principal plane 610 e_2 will be at a position (suchas P1) between the electrodes 610 e-e 2 and 610 e-e3 and theprimary-electron beam 102 can be collimated over there.

The current of the primary-electron beam 102 does not change with theposition of the first principal plane 610 e_2, but its width does andtherefore its current density does too. As the first principal plane 610e_2 is moved closer to the electron source 101, the width of theprimary-electron beam 102 become smaller and the current densitytherefore increases. Accordingly, as the first principal plane 610 m_2moves from P3 to P1 and then to P2, the width of the primary-electronbeam 102 broadens from 102W_P3 to 102W_P1 and then to 102W_P2. If theelectrostatic embodiment 610 e comprises more electrodes placed within alonger area along the optical axis 610 e_1, the current density can bevaried more smoothly within a larger range.

FIG. 13A shows one magnetic embodiment 610 m of the movable condenserlens 610 in FIG. 10 , and shapes of the primary-electron beam 102 whenthe first principal plane 610 m_2 is at the positions P1, P2 and P3respectively. The movable condenser lens 610 m is a compound magneticlens, which comprises two single magnetic lenses 610 m-m 1 and 610 m-m 2aligned with its optical axis 610 m_1. The optical axis 610 m_1 isplaced coincident with the primary optical axis 600_1. The focusingpower and the position of the first principal plane 610 m_2 of theembodiment 610 m vary with the excitation mode of the single magneticlenses 610 m-m 1 and 610 m-m 2. When the excitation of the singlemagnetic lens 610 m-m 2 is set zero, by appropriately setting theexcitation of the single magnetic lens 610 m-m 1, the first principalplane 610 m_2 will be at the position P3 within the magnetic-circuit gapthereof and the primary-electron beam 102 can be collimated over there.When the excitation of the single magnetic lens 610 m-m 1 is set zero,by appropriately setting the excitation of the single magnetic lens 610m-m 2, the first principal plane 610 m_2 will be at the position P2within the magnetic-circuit gap thereof and the primary-electron beam102 can be collimated over there. When the excitations of the singlemagnetic lenses 610 m-m 1 and 610 m-m 2 are not zero, by appropriatelysetting the excitation ratios thereof, the first principal plane 610 m_2will be at a position (such as P1) between the magnetic-circuit gapsthereof and the primary-electron beam 102 can be collimated over there.Accordingly, as the first principal plane 610 m_2 moves from P3 to P1and then to P2, the width of the primary-electron beam 102 broadens from102W_P3 to 102W_P1 and then to 102W_P2. If the magnetic embodiment 610 mcomprises more single magnetic lenses placed within a longer area alongthe optical axis 610 m_1, the current density of the primary-electronbeam 102 can be varied more smoothly within a larger range. To reducethe manufacturing cost, the neighboring magnetic lenses can share themagnetic circuit therebetween, as shown in FIG. 13B.

The movable condenser lens 610 can also be an electromagnetic-compoundlens which comprises multiple annular electrodes and at least one singlemagnetic lens, and its first principal plane can be moved by adjustingthe excitation mode of the annual electrodes and the single magneticlens.

Due to the large current of the primary-electron beam 102, it is easilyperceived that the interactions of the primary electrons may be verystrong if the energies thereof are not high enough. For theprimary-electron beam 102 passing through the main opening of the mainaperture plate 271, only one part is used as the three beamlets(102_1˜102_3), and the other part is not useful. The current of theother part is higher than the total current of the three beamlets, andtherefore generates a stronger Coulomb effect which disturbs the motionsof the primary electrons of the three beamlets and consequentlyincreases the sizes of the three probe spots. Hence it is better to cutoff all or partial portion of the other part as early as possible. Thereare several ways to do so.

Taking the embodiment 300A in FIG. 3 as an example, one way is to placethe beamlet-forming means 321 of the source-conversion unit 320 abovethe condenser lens 210 and close to the electron source 101. In thiscase, the main aperture plate 271 can be removed. Accordingly, FIG. 14shows such an embodiment 700A of the new apparatus. In FIG. 14 , thethree beamlets 102_1, 102_2 and 102_3 respectively pass through thethree beam-limit openings 721_1, 721_2 and 721_3 of the beamlet-formingmeans 721, and the left part of the primary-electron beam 102 is cutoff. The condenser lens 210 collimates the three beamlets into theimage-forming mean 222, and the three micro-deflectors 222_1 d, 222_2 dand 222_3 d deflect the three beamlets in the way same as FIG. 3 . Inthis case, each of the off-axis beam-limit openings (such as 721_2) cannot be aligned with the corresponding micro-deflector (such as 222_2 d)as shown in FIG. 3 , and needs to be placed with respect to theinfluence of the condenser lens 210. The currents of the three beamletscan be changed by varying either the emission (angular intensity) of theelectron source 101 or the sizes of the beam-limit openings 721_1, 721_2and 721_3. The beamlet-forming means 721 can be an electric-conductionplate with multiple through-hole groups, each through-hole groupcomprises three through-holes, and the radial sizes of threethrough-holes in one through-hole group are different from those ofthree through-holes in another through-hole group. Three through-holesin one through-hole group function as the three beam-limit openings(721_1-721_3), and therefore the sizes of the three beam-limit openingscan be changed by using different through-hole groups.

Another way is to use one pre-beamlet-forming means above thesource-conversion unit. Accordingly, FIG. 15A shows such one embodiment800A of the new apparatus, wherein one pre-beamlet-forming means 872with three beamlet-forming apertures 872_1, 872_2 and 872_3 is placedabove the condenser Lens 210, below and close to the main aperture plate271. At first the three beamlet-forming apertures cut the wideprimary-electron beam 102 into three beamlets 102_1, 102_2 and 102_3,and then the beam-limit openings 321_1, 321_2 and 321_3 cut off theperipheral electrons of the beamlets 102_1, 102_2 and 102_3 andtherefore limit the currents thereof. In this case, the currents of thebeamlets 102_1, 102_2 and 102_3 can be changed by varying either theemission of the single electron source or the sizes of the beam-limitopenings or using a movable condenser lens as shown in FIG. 10 . FIG.15B shows such another embodiment 900A of the new apparatus, wherein onepre-beamlet-forming means 972 with three beamlet-forming apertures972_1, 972_2 and 972_3 is placed below the condenser Lens 210. From thereducing Coulomb effect point of view the pre-beamlet-forming means 972in FIG. 15B is not better than the pre-beamlet-forming means 872 in FIG.15A, but it is easier implemented for many obvious reasons andespecially when a magnetic movable condenser lens is used to change thecurrents of the beamlets.

So far, all the foregoing methods for improving the performance of thenew apparatus are individually described on the basis of the embodiment300A in FIG. 3 . Obviously, some or all of these methods can be usedtogether. FIG. 16 shows such an embodiment 1000A of the new apparatus,which uses one pre-beamlet-forming means 872 shown in FIG. 15A to reducethe Coulomb effect due to the primary-electron beam 102, one movablecondenser lens 610 shown in FIG. 10 to vary the currents of the probespots 102_1 s-102_3 s, one image-forming means 422 in FIG. 4 tocompensate the off-axis aberrations of the probe spots. In anotherembodiment (not shown here) similar to the embodiment 1000A, the Coulombeffect due to the primary-electron beam 102 is reduced by employing thepre-beamlet-forming means 972 shown in FIG. 15B.

As well known, the landing energies of the plurality of beamlets can bevaried by changing either or both of the potential of the emitter in theelectron source 101 and the potential of the sample surface 7. Howeveronly varying the potential of the sample surface 7 is advantageousbecause the corresponding adjustment on the source-conversion unit isminor.

In summary, this invention proposes a new multi-beam apparatus whichprovides both high resolution and high throughput for observing a samplein flexibly varying observing conditions, and therefore can function asa yield management tool to inspect and/or review defects/particles onwafers/masks in semiconductor manufacturing industry. In the newapparatus, one condenser collimates or substantially collimates theprimary-electron beam into one source-conversion unit, thesource-conversion unit deflects a plurality of beamlets of theprimary-electron beam towards the optical axis of one objective lens,and the objective lens focuses the plurality of deflected beamlets ontothe sample surface and therefore a plurality of probe spots is formedthereon, wherein the deflection angles of the plurality of deflectedbeamlets are adjusted to reduce the sizes of the plurality of probespots. The currents of the plurality of probe spots can be varied withina large range by changing both of the focusing power and the position ofthe first principal plane of the condenser lens. To further reduce thesizes of the plurality of probe spots, the off-axis aberrations thereofcan be compensated by the source-conversion unit and the Coulomb effectdue to the primary-electron beam can be weakened by placing thebeamlet-forming means of the source conversion unit close to the singleelectron source or using one pre-beamlet-forming means.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. An apparatus for manipulating beamlets of amulti-beam apparatus, the apparatus comprising: a charged particlesource configured to generate a charged particle beam having a sourcecrossover; a plurality of electron optics elements configured to: directa plurality of beamlets generated by the multi-beam apparatus from thecharged particle beam through a primary optical axis of the multi-beamapparatus using at least two different deflection angles, and facilitateforming a plurality of images of the source crossover on a sample; and aplurality of beam-limit openings configured to limit currents of theplurality of beamlets, wherein the plurality of beam-limit openings ispositioned downstream from the plurality of electron optics elements. 2.The apparatus of claim 1, further comprising an objective lens above thesample, and wherein the plurality of electron optics elements is furtherconfigured to: direct the plurality of beamlets to approach a frontfocal point of the objective lens.
 3. The apparatus of claim 2, whereinthe plurality of electron optics elements is further configured todirect the plurality of beamlets to approach or pass through the frontfocal point of the objective lens with different deflection angles,wherein the deflection angles of the plurality of beamlets areindividually set to reduce aberrations of the plurality of images. 4.The apparatus of claim 1, wherein the plurality of electron opticselements is above and aligned with the plurality of beam-limit openingsrespectively.
 5. The apparatus of claim 1, further comprising: anaperture plate between the source crossover and the plurality ofelectron optics elements, configured to suppress a Coulomb effect of thecharged particle beam; and a condenser lens between the aperture plateand the plurality of electron optics elements, configured to focus thecharged particle beam to perpendicularly incident into the plurality ofelectron optics elements.
 6. The apparatus of claim 5, wherein currentsof the plurality of images are varied by using the condenser lens tochange a current density of the charged particle beam.
 7. The apparatusof claim 6, wherein the condenser lens comprises at least one of anannular electrode or a single magnetic lens placed at axial positionsabove and aligned with the plurality of electron optics elements, and atleast one of a voltage of the annular electrodes or an excitation of thesingle magnetic lenses is configured to control the current density. 8.The apparatus of claim 1, wherein each of the plurality of electronoptics elements is a dipole lens.
 9. The apparatus of claim 1, whereineach of the plurality of electron optics elements is a 4-pole lens. 10.The apparatus of claim 1, wherein landing energies of the plurality ofbeamlets are varied by changing a potential of the plurality ofbeamlets.
 11. A non-transitory computer-readable medium storing a set ofinstructions that is executable by at least one processor of anapparatus to cause the apparatus to perform a method for manipulatingbeamlets of a multi-beam apparatus, the method comprising: generating acharged particle beam having a source crossover; directing, using aplurality of electron optics elements, a plurality of beamlets generatedby the multi-beam apparatus from the charged particle beam through aprimary optical axis of the multi-beam apparatus using at least twodifferent deflection angles; facilitate forming a plurality of images ofthe source crossover on a sample; and limiting currents of the pluralityof beamlets by a plurality of beam-limit openings that are positioneddownstream from the plurality of electron optics elements.
 12. Thenon-transitory computer-readable medium of claim 11, wherein directingthe plurality of beamlets towards the sample comprises: directing theplurality of beamlets to approach a front focal point of an objectivelens above the sample.
 13. The non-transitory computer-readable mediumof claim 12, wherein directing the plurality of beamlets to approach thefront focal point of the objective lens above the sample comprises:directing the plurality of beamlets to approach or pass through thefront focal point of the objective lens with different deflectionangles, wherein the deflection angles of the plurality of beamlets areindividually set to reduce aberrations of the plurality of images. 14.The non-transitory computer-readable medium of claim 11, wherein theplurality of electron optics elements is above and aligned with theplurality of beam-limit openings respectively.
 15. The non-transitorycomputer-readable medium of claim 11, wherein the set of instructionsthat is executable by the at least one processor of the apparatus causesthe apparatus to further perform: suppressing a Coulomb effect of thecharged particle beam using an aperture plate between the sourcecrossover and the plurality of electron optics elements; and focusingthe charged particle beam to perpendicularly incident into the pluralityof electron optics elements using a condenser lens between the apertureplate and the plurality of electron optics elements.
 16. Thenon-transitory computer-readable medium of claim 15, wherein the set ofinstructions that is executable by the at least one processor of theapparatus causes the apparatus to further perform: varying currents ofthe plurality of images by using the condenser lens to change a currentdensity of the charged particle beam.
 17. The non-transitorycomputer-readable medium of claim 16, wherein the condenser lenscomprises at least one of an annular electrode or a single magnetic lensplaced at axial positions above and aligned with the plurality ofelectron optics elements, and at least one of a voltage of the annularelectrodes or an excitation of the single magnetic lenses is configuredto control the current density.
 18. The non-transitory computer-readablemedium of claim 11, wherein each of the plurality of electron opticselements is a dipole lens.
 19. The non-transitory computer-readablemedium of claim 11, wherein each of the plurality of electron opticselements is a 4-pole lens.
 20. The non-transitory computer-readablemedium of claim 11, wherein the set of instructions that is executableby the at least one processor of the apparatus causes the apparatus tofurther perform: varying landing energies of the plurality of beamletsby changing a potential of the plurality of beamlets.